1. Field of the Invention
The present invention relates to a positive electrode active material powder for a lithium secondary battery, which has a large volume capacity density and high safety and is excellent in the charge and discharge cyclic durability, a positive electrode for a lithium secondary battery containing such a powder, and a lithium secondary battery.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-cobalt composite oxide (LiCoO2) as a positive electrode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a positive electrode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic properties such as gradual reduction of the battery discharge capacity due to repetitive charge and discharge cycle, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
In order to solve these problems, it has been proposed in patent document 1 that the average particle size of LiCoO2 as a positive electrode active material, be from 3 to 9 am, the volume occupied by a group of particles having a particle size of from 3 to 15 μm, be at least 75% of the total volume, and the intensity ratio of the diffraction peaks at 2θ=about 19° and 45° as measured by means of X-ray diffraction using CuKα as a radiation source, be of a specific value, so that it becomes an active material excellent in the coating properties, the self-discharge properties and the cyclic properties. Further, in the patent document 1, it has been proposed that the positive electrode active material is preferably one which does not substantially have such a particle size distribution that the particle size of LiCoO2 is 1 μm or smaller or 25 μm or larger. With such a positive electrode active material, the coating properties and the cyclic properties have been improved, but, the safety, the volume capacity density and the weight capacity density, have not yet been fully satisfactory.
Further, in order to improve the weight capacity density and the charge and discharge cyclic properties of the positive electrode, patent document 2 proposes a positive electrode active material wherein the average particle size of lithium composite oxide particles is from 0.1 to 50 μm, and at least two peaks are present in the particle size distribution. Further, it has been proposed to mix two types of positive electrode active materials having different average particle sizes to prepare a positive electrode active material wherein at least two peaks are present in the particle size distribution. In such a proposal, there may be a case where the weight capacity density and the charge and discharge cyclic properties of the positive electrode can be improved, but on the other hand, there is a complication that the positive electrode material powders having two types of particle size distributions have to be produced, and one satisfying all of the volume capacity density, the safety, the coating uniformity, the weight capacity density and the cyclic properties of the positive electrode, has not yet been obtained.
Further, patent document 3 proposes to replace 5 to 35% of Co atoms in LiCoO2 with W, Mn, Ta, Ti or Nb to improve the cyclic properties. Further, patent document 4 proposes to use hexagonal LiCoO2 as a positive electrode active material to improve the cyclic properties, wherein the c axis length of the lattice constant is at most 14.051 Å, and the crystal lattice size of (110) direction of the crystal lattice is from 45 to 100 nm.
Further, patent document 5 proposes a granular lithium composite oxide having a compression breaking strength per grain of from 0.1 to 1.0 gf, which has the formula LixNi1-y-zCoyMezO2 (wherein 0<x<1.1, 0<y≦0.6, and 0≦z≦0.6) and which is an agglomerated granular lithium composite oxide formed by agglomeration of a fine powder. However, such a composite oxide has a problem that the safety is poor, and the large current discharge properties are inferior. Besides, with such a small range of compression breaking strength, it is not possible to obtain a lithium composite oxide having adequately satisfactory properties with respect to e.g. the volume capacity density, the safety, the cyclic properties and the large current discharge properties.                Patent Document 1: JP-A-6-243897        Patent Document 2: JP-A-2000-82466        Patent Document 3: JP-A-3-201368        Patent Document 4: JP-A-10-312805        Patent Document 5: JP-A-2001-80920        